Earthquakes in the Eagle Ford shale region of Texas are likely being triggered by increased oil extraction, according to new research from the University of Texas at Austin.

A two-year survey of seismic activity links small quakes in South Texas largely to the upswing in the production of oil and brackish water that flows up alongside hydrocarbons.

However, researchers concluded the quakes were not related to hydraulic fracturing. Nor do disposal wells, where companies discard hydraulic fracturing fluids and brackish water found underground, appear to trigger the quakes.

Previous studies have linked earthquakes to the disposal of fracking fluids in deep wells in other regions, including in other parts of Texas and in Ohio.